The first broad objective of this research is to obtain quantitatively accurate structure of fully hydrated lipid bilayers with and without added components such as peptides. Such characterization is essential to test the hypothesis that the differences in lipid composition in different biomembranes in the body are related to structural differences in the underlying lipid bilayers that are required for membrane protein functionality and healthy cell function. The results of the proposed work will guide and evaluate molecular dynamics simulations. The second broad objective is to obtain quantitatively accurate interactions between bilayers, which are useful for understanding membrane adhesion and fusion. Study of these interactions involves nanoscale properties of membranes, such as the bending modulus, area compressibility and thermal expansion coefficient, which are input data for considering membrane morphology and cell shape changes. Our primary technique is x-ray diffraction which will be complemented by our volumetric measurements, as well as with data from other laboratories, such as NMR, neutron diffraction and molecular dynamics simulations. We have made a new breakthrough that enables study of oriented membrane preparations in the fully hydrated, biologically relevant fluid phase. Oriented samples have the advantage that the data extend to higher q, so more complete structures can be obtained. We have developed an analysis that extracts both structural and fluctuational information from these data to address both of our broad objectives. This novel method will be appropriate for studying a variety of biomembrane systems. We propose to apply it to the HIV fusion peptide to determine the depth of penetration of the N-terminus into the hydrophobic region of lipid bilayers.